I too believe it will be between 780 an 780Ti - $500 price tag makes sense that it will be that, since otherwise that would instantly kill off their 780Ti and Titans profits.
I am sure that they are simply going to artificially cripple 880s so that 780Ti would still make sense to toss $700 for (or titans ofc), I can already see where - 256bit VRAM bus with the same clock as 780s... That's 224GB/s bandwidth, subpar by current standards and will affect gaming at decent AA settings.
That may also explain why it benchmarks higher than Ti - probably with no AA it will be better than 780Ti, but the moment sufficient AA is in, it will get outperformed by 780Ti and benchmark just shows no AA performance.